Open core sandwich-structure



July 14, %70 R. c. BRUNER OPEN CORE SANDWICH-STRUCTURE 3 Sheets-Sheet l Filed July 5, 1967 POWER PLAT\ NG SUPDLY m T N E V m ?AL/ H C. ERA/NE? 14, 1%70 R. c. BRUNER 3 OPEN CORE SANDWICH-STRUCTURE 3 Sheets-Sheet 2 ?iled July 3, 1967 INVENTOR. PAA P/-/ C. BRUA/E? BY/JW Juy M, 1970 R. c. BRUNER 3,520,'357

OPEN CORE SANDWICH-STRUCTURE Filed July 5, 1967 5 Sheets-Sheet ;s

I r\ I /8e 304,

/ e I t a /Zf I z\ INVENTOR, /QL H C'. &PU/VEQ GENT United States Parent O 3,520,357 OPEN CORE SANDWICH-STRUCTURE Ralph C. Bruner, Tulsa, Okla., assignor to North American Rockwell Corporation, a corporation of Delaware Filed July 3, 1967, Ser. No. 650,942 Int. Cl. C23b 7/22; F28f 3/12 U.S. Cl. 165--166 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND This invention relates to a structural-element having low weight and high strength; and more particularly to a modification of the sandwich-type structural-element wherein a core is sandwiched between, and aflixed to, two external skin-sheets.

At present the most widely used Sandwich-type structural-element is the well known honeycomb' panelwherein a plurality of sheets of material are juxtapostioned in such a way as to form a core having a plurality of heXagonally-shaped cells, and two external skin-sheets are positioned and afi'ixed to the honeycomb core in such a manner as to form a Sandwich that closes and isolates each hexagonal cell. Structures of this type are extremely lightweight and strong, and have found wide usage. These uses have included the floors, walls, wings, and-in many cases-combined wings and fuel tanks of an aircraft; and have also included "heat exchangers wherein a heating or a cooling fluid (liquid or gas) flows through the core, and transfers heat to the honeycomb walls and skinsheets.

In these latter tank and fluid-flow usages, it becomes necessary-because of the orientation of the sealed honeycomb cells--to perforate the cell walls in order to permit the fluid to flow from one cell to another. This requirement for perforations tends to reduce the high-strength advantage of the honeycomb cell-because the perforations must be large enough to permit the flow of fluid; r

and yet, any perforations-however small-tend to Weaken the structure.

Many attempts have been made to provide a more satisfactory Sandwich-type structural-element; that is, a structural-element with an equivalent or strength-to-weight ratio that also permts better flow of fluid, greater strength or flexibility, etc. These prior-art structural-elements have generally sought to replace the honeycomb-core with other cores that have been formed in one of two ways. The first way has been to form the core by deforming or stretchng a sheet to have shaped protrusions. The disadvantage of this technique is that high-strength metals can generally be stretched or deformed to only a limited eXtent, thus introducing limited wall-thickness, brittleness, metal-tearing, etc. An alternative way of forming the cores has been to fabricate separate protrusions of suitable size and shape, and to aflix these by welding or brazing to suitable base-sheets. This particular arrangement has the disadvantage that it requires more manufacturing processes, and tends to be unreliable because of the inability to perfectly perform the numerous welding and/ or brazing Operations.

improved t 3,520',357 Patented July 14, 1970 Another disadvantage of these prior-art techniques is that the metal-forming processes generally produce walls that tend to be of unform or reducing thckness; and, therefore, certain wall areas tend to be more highly stressed under load than other wall areas. Structuralelements made by the above techniques have still another deciency in common; this being the fact that they tend to 'be inherently planar structures. In order to achieve curved panels, they must be formed planar, and then bent to the desired shape; but this bending process introduces stresses and strains, and tends to buckle and weaken the material of which the Sandwich-type structural-element is made.

OBJECTS AND DRAWINGS It is therefore an object of the present invention to provide an improved, lightweight structure.

It is another object of the invention to provide an impoved core that can be electroformed in any desired s ape.

It is another object of the present invention to provide a structure core-Component wherein the wall-thickness of the core is suitably tapered to have equivalent strength at all portions thereof.

The attainment of these objects, and others, will be realized from the teachings of the following detailed description; taken in conjunctior with the drawings, of which FIG. 1 shows a single-panel structure;

FIG. 2 shows a method of electroforming the core- Component;

FIGS. 3a and 3b show typical resultant cross-sectional areas produced by tapering the wall-thickness of the core- Component;

FIG. 4 shows a double-panel structure;

FIG. 5 shows a multi-panel structure;

FIG. 6 shows another multi-panel; and

FIG. 7 shows a double-curved structure.

SYNOPSIS Broadly speaking, the disclosed structural-element comprises a core-Component that is electroformed in such a way as to result in a base-sheet having integrally-formed configurations protruding therefrom; these protrusions preferably being in the form of frusto-cones, trusto-polyhedrons, or frusto-pseudo-spherical configurations, and being designated "apixially shaped.

DESCRIPTION The basic inventive concept will be understood from FIG. 1, wherein a single-panel structure 10 is comprised of a core-component 12 and two skin-sheets 14a and 14b. As shown, core 12 comprises a base-sheet 16 and apixical configurations 18-shown as frusto-cones-protruding from a base-sheet 16. Since, as indicated previously, corecomponent 12 is produced by electroforming (to be discussed later), a base-sheet 16 and configurations 18 form a contnuous integral structure; and the configurations 18 may be of any desired size, shape, height, angle, etc. Structure 10 is unitized by means of brazing, Welding, adhesives, etc.; and provides the desired lightness associated with the desired strength. Moreover, the array of protrusions 18 permits conduit, pipes, tubing, wires, etc. to be readily inserted into-and through-the structure 10. Moreover, the protrusion-array provides a relatively unimpeded path for fluid-flow along a channel formed by the two skin-sheets 1411 and 14b, the protrusions 18` being positioned to provide either a substantially laminar flow, or mildly-turbulent flow, for conducting heat into or out of the ow.

As ndicated previously, core-Component 12 is electroformed, and this process may be understood from FIG. 2. Here core-Component 12 is being forned on a matrix 20 which, in association with electrode 22 and a plating power-supply 24, acts in a well known manner to deposit metal from plating solution 26 onto matrix 20. In this way, core-Component 12 assumes a shape and size determined by matrix 20. Due to the electroforming process, a continuum of metal is deposited on matrix 20, so that the basesheet 16 is an integral continuous sheet, and is integral and continuous with protrusions 18, to form an integral continuous core-component 12.

As is known to those skilled in the art, the matrix 20- or its surface-must be electrically conductive; and, in the case of a non-conductive matrix material, a surfaceconductive coating-indicated at 28-can be produced by the well known electroless-plating technique.

The process of electroforming is well known and widely used, and is described in many publications such as "Principles of Electroplating and Electroforming by Blum and Hogaboom; Metal Finishing Guide Book published by the Metals and Plastics Publications, Inc. of Westwood, N.J.; and "Electroplating Engineering Handbook by Graham. As is well known, electroforming is a form of electroplating wherein the plated material deposits upon a matrix in such a manner as to produce an object corresponding to the size, shape, and configuration of the matrix, the matrix being eventually removed from the electroformed workpiece, to leave a metal shell of the desired shape and dimensions. It is well known, that by suitable positioning of the electrodes, suitable composition of the plating solution, suitable positioning of so-called "rubber" electrodes, and suitable control of the plating current, the electroforming process may be caused to produce a workpiece whose thickness varies in a desired manner relative to the configuration of the matrix; and this thickness-control is discussed and illustrated on page 62 of the 1955 issue of the Electroplating Engineering Handbook cited above. T hus, the disclosed tapered wall-thickness of protrusions 18 may be readily electroformed by techniques well known to those skilled in the art. p

It has been found that a satisfactory "peelable matrix may be formed of the material designated as Flexible TC459 Black Epoxy manufactured by the Chemical Division, Electronic Production and Development, Inc., 509 North Prairie Ave., Hawthorne, Calif. However, because of the apixical shape of the protrusions, a peelable matrix is not essential; but where desired may also be formed of any material that can be cast, molded, or machined to the desired configuration. These materials would include poly- V urethanes, polysiulfides, various rubbers, silicones, etc.

It should be noted that becasue protrusions 18 are produced by the electroforming process, they may be as high and as sharply-angled as desired. This is a tremendous advantage over prior-art metal shaping and metal Stretching techniques wherein, because of limitations of highstrength materials, the maximum height-to-diameter rato is equal to a numerical value of about one. Much larger height/ diameter ratios may be achieved by matrix design and electroforming; thus giving wider latitude to the designer.

Referring again to FIG. 2, it will be noted that the walls of configurations 18 have a tapered thickness that becomes thicker near the apex. The advantage of this tapered thickness will be recognized from FIG. 3. Here the basal cross-section (cross-section parallel to the base) of FIG. 3a indicates a smaller outer-diameter than the basal cross-section shown in FIG. 3b. However, due to the tapered-thickness concept, the basal cross-sectional area of the wall 18 shown in FIG. 3a is the same as the basal cross-sectional area of the wall 18 shown in FIG. 3b; in this way, permitting each basal cross-section to provide the same amount of material, for equalizing stresses and strains under load. Electroformed cores have proven satisfactory.

Under some conditions-for desired spacing, dimensions, Volume, fluid-flow, flexibility, rigidity, etc., a double-panel structure may be desirable, and this is shown in FIG. 4. Here structure 10a is formed of two panels; an upper panel 12a and a lower panel 1217, each having its own base-sheet 16a, 1617 and configurations 18a and 18b. In FIG. 4, the two panels 1211 and 1217 are aflixed in an apixical relationship, with the apixical portions of configurations 18a and 18b in apixical contact. As previously indicated, skin-sheets Ma and 14b are aflixed to the outside of the structure, in this case to base-sheets 1611 and lb. In the structure of FIG. 4, the spacing between the skin-sheets Ida and 14b may be much larger than previously indicated, and may thus accommodate dtterent amounts of fluid-flow, pressure, conduits, etc.

FIG. 5 shows another multi-panel structure 10b that permits the containment or simultaneous or counterfiow of two dissimilar gases or liquids. Structure 1017 is similar to that previously shown, except that a Separator sheet 30 is aixed in position between adjacent apices of the configurations 180 and 18d.

It should be noted that structure mb of FIG. 5 is shown as part of a cylindrical wall, the individual corecomponents having been electroformed in this configuration, so that the cylindrical single-curved structural-element does not have any inherent strains.

FIG. 6 shows still another multi-panel structure comprising two structural-elements 12@ and 12f containing base-sheets 16e and 161", and protrusions 18@ and 18f respectively. Skin sheet 146 and 14f complete the structure. Structural element lc is similar to the previous arrangement, except that panels `12@ and 127 are base-tobase rather than apex-to-apex. This is clear from the proximity of base-sheets 162 and 16f in FIG. 6, and the distance of base-sheets and 1611 in FIG. 5. The separator sheet Sea may be either used or omitted, depending upon requirements.

FIG. 7 shows a structural-element ltld of the disclosed type as used in a domed configuration, this being a double-curved structure rather than the single-curved structures previously discussed. Since the double-curved arrangement of FIG. 7 is also electroformed, it is free of inherent strains; and may be made integral with singlecurved and/or planar section. This is a tremendous advantage over prior-art structures. As previously indicated, one of the disadvantages of prior-art attempts to produce a satisfactory structure of this type, was their inherent tendency to form planar structures; these prior-art structures taking this curved formaton only at the expense of much rework and the introduction of highly strained portions.

Thus, the disclosed invention permits the panels to be electroformed on a matrix that is suitably sized, shaped, and curved so that the resultant panel-and therefore the structuremay be cylindrical (single-curved) or convex, concave, or compound-curved (double-curved). In this way, the disclosed structure has improved configuration advantages over prior-art structures.

In the above way, the disclosed structure provides a novel lightweight low-density structure that may be used where lightweight and high strength are desired; and still prov i de relatively unimpeded insertion of pipes, or flow of liquids and gases when used for means such as a fuel tank, heat exchanger, and the like.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only, and is not to be taken by way of limitation; the spirit and scope of this invention being limited only by the terms of the appended claims.

What is claimed is:

1. A core for a structural element of the type having a lightweight core that is sandwiched between external skin-sheets, said core comprising a continuous integral base-sheet with a plurality of integral electroformed References Cited apixally-shaped configurations protruding from said UNITED STATES PATENTS base-sheet, said configurations having a tapered Wall thickness, the wall of said configrations having substan- 2986338 1/1935 Harris 204-9 tally the same cross-sectional area at each basal cross 5 11963089 7/1965 Stoycos 204-281 section of said configuration, said configurations having &445348 5/1969 Aske 204 11 a height/ diameter ratio greater than one. FOREIGN PATENTS 2. A dual-flow combination comprising: a pair of cores of the type described in claim 1; 7/1958 Great Bntama Separator sheet; means afiixing one of said cores to one side of said 10 JOHN MACK? Pnmary Exammr Separator sheet; and T. TUFARIELLO, Assistant Examine' means afxing the other of said cores to the other side of said Separator sheet for causing each core to ac- U.S. Cl. X.R.

commodate its individual flow. 15 11 

